Excerpt
“Unnatural Selection: Choosing Boys Over Girls and the Consequences of a World Full of Men”
By Mara Hvistendahl

For as long as they have counted births, demographers have noted that on average 105 boys are born for every 100 girls. This is our natural sex ratio at birth. The ratio can vary slightly in certain conditions and from one geographic region to the next. More boys are born after wars. More girls are born around the equator, for reasons we don’t yet understand. But in general the sex ratio at birth hovers around 105.

So is our population male-dominated from the start? To the contrary: that more boys are born is itself a form of balance, neatly making up for the fact that males are more likely to die young. That extra 5 percent of boy babies compensates, as the German statistician Johann Peter Süssmilch observed in 1741, “for the higher male losses due to the recklessness of boys, to exhaustion, to dangerous tasks, to war, to sailing and emigration, thus maintaining the balance between the sexes so that everyone can find a spouse at the appropriate time of marriage.” While today males are less likely to die from sailing, exhaustion, or migration, they still account for the majority of soldiers throughout the world. They also disproportionately expose themselves to threats like smoking—a man’s pursuit in many countries—or riding motorcycles without wearing a helmet. Boys outnumber girls at birth because men outnumber women in early deaths.

Süssmilch, who was also a priest, was an early proponent of intelligent design; he concluded this natural check was the work of a meticulous creator. (The book in which he put forth his theory was titled The Divine Order as Derived from Demography.)3When Charles Darwin looked into the sex ratio at birth a century later, he intuited that a balanced number of males and females instead connected somehow to evolution. Trends in human populations, Darwin noted, paralleled those found in the animal world. But that raised a question: What then was the purpose of the intense battles for mates among many species? To witness “two males fighting for the possession of the female, or several male birds displaying their gorgeous plumage, and performing strange antics before an assembled body of females,” as Darwin wrote in The Descent of Man, it was clear that a fierce evolutionary competition was at work. This competition was perhaps most evident in the peacock’s feathers: the colorful plumes would make sense if, as a rule, the sex ratio were skewed. If peahens were generally scarce, the male birds’ adornment would be a feature they had developed over generations to boost their chances of passing on their genes. A balanced sex ratio meant even the ugliest and most pitiful peacock had hope of finding a peahen.

But after extensive correspondence with farmers, shepherds, and biologists—Darwin even dutifully tallied sex ratios among English racehorses—the naturalist determined most species were in fact balanced. “After investigating, as far as possible, the numerical proportion of the sexes,” he wrote, “I do not believe that any great inequality in number commonly exists.”

Darwin went back and forth on exactly how a balanced sex ratio could be reconciled with his theory of natural selection, coming very close to a solution in the first edition of The Descent of Man and then retracting it in the second edition. “I now see that the whole problem is so intricate that it is safer to leave its solution for the future,” he wrote. And yet the naturalist surmised that balanced sex ratios were somehow critical to species survival.

In 1930 the English scientist Ronald A. Fisher arrived at the explanation that had eluded Darwin. Fisher’s theory works like this in humans: if male births become less common, men have better mating prospects than women. People with an assumed genetic disposition to have boys then have an advantage in passing on their genes. Put more simply, parents of sons have more grandchildren than parents of daughters. As the overall sex ratio approaches equilibrium, however, the advantage of producing sons disappears, and the sex ratio at birth balances out. (Unfortunately, this mechanism does not work on skewed sex ratios of the sort seen in Asia today.) Fisher was also an enthusiastic eugenicist who believed in sterilizing the “unfit.” With John Maynard Keynes, he was among the founding members of the Cambridge University Eugenics Society. But he enshrined in evolutionary biology the notion that sex ratios are naturally balanced. Today a 1:1 sex ratio is called “Fisherian.”